Mixing vessels may be used in a variety of industrial applications. They may be used as precipitators in alumina production, anaerobic digesters in waste water treatment, and in many other applications. For example, in alumina production, two predominant mixing technologies may typically be used: draft tube mixers and mechanical agitators with impellers on very long shafts.
Draft tube mechanical mixers typically provide vertical circulation of suspended solid particles by having a pumping impeller inside of the tube that reaches deep into the mixing vessel. The vessel and draft tube usually are free of obstructions, or alumina may precipitate on the vessel walls in zones of low flow velocity. In order to prevent this scaling on the interior of the vessel walls, the vessels are typically equipped with baffles. Unfortunately, these baffles prevent inhibit or prevent rotation of the liquid inside the vessel.
Even with baffles on the interior of the vessel walls, precipitate may eventually build up on the baffles and vessel walls. Such precipitator vessels must be periodically taken off-line for cleaning of alumina deposits. If the vessel is not cleaned often enough, the weight of the precipitated material may cause the collapse of the internal baffle structures. However, cleaning often causes disruption to production cycles, and it may be costly.
Also, draft tube precipitators typically must be operated at high flow velocities to minimize precipitate build-up on the baffles. Therefore, the impeller blade speed must also be high, and that may result in high erosion rates at the impeller blade tips. Eroded impeller blades may require frequent impeller replacement.
As an alternative to draft tube mixers, mixers with long impeller shafts (which may submerge the impeller blades far below the liquid surface) may also be used. These vessels are sometimes operated without baffles, because the mixer may induce a predominantly swirling flow with a small radial velocity component. Therefore, the propensity for scaling at the vessel wall is minimized, but due to low turbulence in the vessel center, crystals may precipitate on the slowly-rotating impeller shaft and impeller blades. This build-up may require periodically taking the vessel off-line for cleaning of precipitate deposits on the impeller assembly.
Another method of mixing liquids and solids is described in U.S. Pat. No. 6,467,947. This mixing apparatus contains a short impeller shaft and radial impeller blades, with the impeller blades located adjacent to the surface of the liquid. The rotational motion of the impeller blades induces a swirling motion in the vessel allowing for suspension of solid particles. However, the use of radial impeller blades may make particle suspension inefficient, from an energy standpoint. Also, this method may require a high mixer speed, which may cause significant erosion of the impeller blades.
The present invention may provide a mixing apparatus and method for continuous mixing in a vessel that minimizes vessel wall and impeller assembly precipitate build-up with limited impeller blade erosion for longer service between maintenance activities.